I uninstalled TestNG from my eclipse and then i tried installing it again in this mentioned steps
Help>Install new software>Selecting TestNG Checkbox>Clicking Next and Finish buttons.
Then its showing the Error.
I hope good to clean the eclipse once and try to install again.
To start eclipse with clean, Edit the eclipse.ini file located in your Eclipse install directory and insert -clean as the first line. Further more info on it here
Note: If you keep clean option, then eclipse always start deleting cache etc so it may take some time.
Then install TestNG from Marketplace( as already tried another ways)
Thank You, Murali